System for monitoring gas insulated substations

ABSTRACT

A diagnostic measuring system (20) for gas insulated electrical substations having UHF couplers (11) fitted to bus chambers (12) monitors for partial discharge events within the chamber (12) to identify the nature and location of an emergent breakdown condition. The system (20) identifies each partial discharge event but characterizes it by amplitude and its time of occurrence with respect to the start of the 50 Hz power frequency wave. The characterized events are communicated over an optical fiber link (13) configured as a token passing ring to an analyzer (25) where a comparison is made with stored historic data characterizing known events. The analyzer (25) provides a self checking routine for the system (20). A microprocessor (36) forming part of a unit (21) associated with a plurality of couplers (11) provides data compression of events identified from these couplers.

This invention relates to diagnostic measurements in gas insulatedelectrical substations.

Gas insulated substations (GIS) are utilised in many electricitytransmission networks and are advantageous because of their beingcompact in size. In a GIS the conductors and circuit devices, such astransformers and switchgear, are housed within pressure vesselscontaining an atmosphere of sulphur hexafluoride at elevated pressure.The pressure vessels are typically made of steel and, electrically, areheld at earth. In a 3-phase network the conductors of the individualphases are separately encased in pressure vessels and are mechanicallymounted therein via insulators. The pressure vessels typically compriseinterconnected tubular members which may have side-wall inspectionports.

Fault conditions which arise in the GIS lead to lengthy disconnectionperiods due to the mechanical complexity of the GIS and there istherefore a requirement to provide diagnostic measurements during inservice use of the GIS in order to predict the possibility of a faultcondition and to enable corrective action at a planned and convenienttime. These potential faults almost always show partial electricaldischarge activity before breakdown occurs and this discharge activitycan be sensed from the consequential ultra high frequency resonancemodes established in the pressure vessels. Accordingly the UHF modes canbe sensed by couplers built into the pressure vessels at the inspectionports as has been proposed in the article entitled "Diagnosticmeasurements at UHF in gas insulated substations" published in PROC IEEVol 135 Pt.C No. 2, March 1988. However, this proposal requires the useof a spectrum analyser connected to the coupler output and humaninterpretation of the analyser result.

The present invention provides a diagnostic measurement system for gasinsulated electrical substations, the system comprising means forautomatically monitoring UHF couplers fitted to pressure vessels in thesubstation, the monitoring means comprising means for identifyingindividual partial discharge events, means for characterising identifiedevents, means for communicating characterised events from a plurality ofcouplers to an analysing means, the analysing means comprisingpre-stored data representative of known pre-fault conditions, andcomparative means for comparing the pre-stored data with thecommunicated characterised events, and for issuing a warning signal inthe event of identity.

The identifying and characterising means preferably form part of amicroprocessor sub-system associated with a plurality of couplers. Thesub-system preferably comprises means for compressing a plurality ofcharacterised events into a single data word for communicating to theanalysing means. The communicating means preferably comprises an opticalfibre loop. The analysing means preferably comprises means forsignalling the sub-system to communicate characterised events from aparticular coupler without data compression.

An embodiment of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIGS. 1 and 2 schematically illustrate a gas insulated electricalsubstation and a diagnostic measurement system coupled thereto;

FIGS. 3-5 illustrate components of the measurement system; and

FIGS. 6A-6G illustrates typical waveforms occurring at the components ofFIGS. 3-5.

A gas insulated substation (GIS) 10 and a diagnostic measurement system20 coupled thereto are shown schematically in FIGS. 1 and 2 wherein theUHF signals are taken from the GIS 10 using couplers 11 mounted on theinside of hatch covers formed on bus chambers 12. The couplers 11 havebeen designed to have a good high frequency performance, and aremechanically very robust and reliable. They may easily be fittedretrospectively to a GIS already in service.

The standard is to detect a 2 mm particle moving anywhere within theGIS, because this is below the size which might lead to breakdown.Because of the signal attenuation at these high frequencies, thecouplers need to be fitted at not much more than 20 m intervals alongthe bus chambers 12. A typical 420 kV GIS will therefore need perhaps20-30 three-phase sets of couplers in all.

In the continuous monitor 20, the couplers are connected to detectionand pulse processing units 21 which may be implemented using amicroprocessor and designed to respond to signals in the UHF region. Theunits 21 contain circuits for signal amplification, level detection andtiming of the pulse with respect to the start of the 50 Hz powerfrequency wave; they are well screened externally, and also protectedagainst the transient overvoltages arising, for example, from theoperation of a nearby disconnector.

The processing units 21 digitise the outputs of each 3-phase set ofcouplers 11 and handle the digitised outputs together at a network node21 D in an optical fiber loop network 13, with a unique addresseffectively assigned to each coupler 11. The network could contain, say,30 nodes, and is configured as a token passing ring. It allows datatransfer at 19,200 baud in one direction only using polled, exception ortimed transmissions. The nodes run application specific software inconjunction with a standard shell for communications and specificregister management. Any node may act as master on the network, with themain functions of starting the network operation and, for example,handling periodic token frame transmissions.

A host computer 25 may be connected as part of the network 13, and willoften act as the master. It is used to store the coupler data for laterprocessing and analysis, and to interrogate any node during the systemdiagnosis.

When a partial discharge (PD) occurs and the unit 21 is operating in itsexception mode, it flags the system and a data token is transmitted onthe network 13. This contains information on the PD amplitude, its pointon wave, and the address of the coupler 11 at which the PD was detected.The token is passed to the master 25 and the data stored. Unless the PDsignal level is very low, it will be detected at virtually the same timeby several couplers 11 on either side of the discharge source, but thenetwork protocol prevents token crash by storing data at the units 21 ona time-tagged basis for later transmission and processing in time orderby the master 25.

On arrival at the master 25, the data is time tagged so that orderedprocessing may be undertaken. The master 25 also continuously monitorsand can display the data, and initiates preprogrammed procedures shouldany threshold of signal amplitude or number of PD pulses be exceeded.

If the GIS is to be unattended, an auto-dial/auto-answer modem 26 wouldalso be included in the network 13. This could replace the PC 25 as thenetwork master, and initiate alarm calls to other locations such as theutility's headquarters 27.

In general the advantages of this system 20 are that the optical fibrenetwork 13 allows interference-free communications within the noisyenvironment of the GIS, and the ring network 13 simplifies theinstallation of the system. The low baud rate reduces the chance oftoken loss or corruption, and together these measures give highreliability to the communications and monitoring system 20.

The integrity of the node electronics and optical fiber loop ismonitored when the network operates on either timed or polled modes.Instructions can be passed from the nost, either on site 25 or remotelyvia the modem 26, to inject test pulses into any selected coupler 11 orits processing unit 21 so that its response may be checked.Alternatively, a self-check facility for all couplers may be used anddiagnostic polling carried out at preprogrammed intervals.

In addition to the above checks, the nodes are programmed to transmit atest signal to the master 25 at timed intervals. Any failure to receivethe signal indicates a break in the optical fiber ring 13, or a failureat a node. Additional checks would enable this to be located quickly.

The digitised coupler signals are normally preprocessed at the nodes ofthe units 21 to compress the data for transmission. This enables thecomplete GIS to be monitored, and the onset of a PD detected. Thecompressed PD signals are stored in a database at the master 25 readyfor analysis. Initially this may show, for example, the most activecoupler 11, with the discharge level, repetition rate and point-on-waveof the discharge. Although the database is continuously updated,skeleton data are retained so that if necessary the development of thedischarge over a long period of time may be seen.

Having been informed of a developing discharge via the modem link 26, anengineer at his headquarters location 27 may request more detailedinformation from a particular coupler 11. In this case the othercouplers 11 would be data disconnected, and the envelopes of theindividual pulses from the selected coupler received directly andwithout preprocessing. They could then be displayed directly in realtime--so that effectively the engineer would be able to examine thecouplers in turn from his office. The versatility of this system allowsany desired level of processing to be undertaken, and the systemconfigured to suit individual requirements.

An active discharge in a GIS can generate vast amounts of data, and itis recognised that a site engineer may not always have the backgroundwhich would enable him to decide what action to take. The detailedcharacteristics of various discharge conditions have already beenrecorded in laboratory tests, and the times before they lead to completebreakdown found. With this in comparative use at the host 25, theengineer will be presented automatically with the information he needs,which is:

the name of the GIS, if more than one is being monitored

the phase and approximate location of the discharging condition

the nature of the condition (free particle, disconnected shield, etc.)

the estimated time before complete breakdown.

The capacity of the optical network 13 is such that up to 100 additionalchannels are available to monitor other parameters in the GIS. Thesecould be used to record any quantity of interest measured by atransducer as indicated schematically at 14; for example gas density,circuit breaker travel and contact wear.

In more detail, as is shown in FIGS. 3,4 and 5 of the drawings with theaccompanying signal waveforms of FIGS. 6A-6G a three-channel unit 21 isshown in FIG. 3 connected to three couplers 11 associated withrespective phases of a 3-phase GIS. This arrangement is mechanicallyadvantageous because in a 3-phase system the three couplers are usuallyclosely grouped. The input to the unit 21 is protected in two stagesagainst transient over voltages. First, a co-axial stub fitted at thecoupler 11 acts as a high pass filter to transmit only the UHF signalsof interest. Second, the input stage of the unit 21 comprises an inputprotection circuit 21A which utilises gas filled discharge arresters orhigh speed Schottky diodes to protect against exceedingly largedischarge spikes. The output of circuit 21A is delivered to a microwaveamplification and detection circuit 21B and which is shown moreparticularly in FIG. 4. Circuit 21B is designed to accept pulse signalsin the frequency range 500 MHz to 1300 MHz which is in the UHF or lowermicrowave range and with a dynamic range of 75 dB. The dynamic rangespans the lowest amplitude input signal level which typically representsa corona discharge up to the highest amplitude signal level whichtypically represents a floating electrode. Discharges from movingparticles have an amplitude range within this dynamic range.

The purpose of the FIG. 4 circuit is to output a pulse (signal E) shownin FIG. 6F representing the envelope G of the UHF input pulse (signal A)shown and FIG. 6A. To achieve this and take account of the extremelylarge dynamic range each input pulse is processed through a laddernetwork of microwave diode limiting and amplification stages 23 theoutput of each being rectified and low pass filtered at 24 to produce aplurality of signals each as represented by signal C. Signal B isillustrated in FIGS. 6B and 6C. FIG. 6B shows signal B if amplitudegreater than diode limiting. FIG. 6C shows signal B if amplitude is lessthan the diode limiting. At least the first stage of the ladder networkwill produce the signal C shown in FIG. 6D but successive stages of theladder network may produce no signal C output due to the low signallevel in these stages. All of these signals are summed by a videoamplifier 28 to provide signal D shown in FIG. 6E and subsequentlyfiltered at 26 to provide the envelope signal E. Accordingly signal Ehas an amplitude which is correlated with that of signal A but issmoothed to facilitate derivation of subsequent amplitude and timemeasurements. Although the ladder network loses the amplitude of verylarge signals due to diode limiting it may be inferred by measurementsderived from a number of adjacent couplers.

Envelope signal E is delivered from circuit 21B to a data acquisitioncircuit 21C which is shown in greater detail in FIG. 5. The FIG. 5circuit is shown for all three phases of the GIS whereas in FIG. 4 thecircuit is applicable to only a single phase. In FIG. 5 the receivedsignal E passes through an isolation circuit 38 and is peak detected andheld by circuits 31,32 to enable amplitude measurement in digital formby analogue to digital converter circuit 33. The time occurrence of theleading edge of the received microwave signal is measured by circuit 34with respect to a 50 Hz reference input signal derived from the GIS toenable the point-on-wave identification of the partial discharge eventto be determined, and circuits 33 and 34 are both gated by aleading-edge detection circuit 35. Circuit 35 receives signal F shown inFIG. 6G being the output of the peak hold circuits 31 and provides agating signal provided that the peak level of signal F exceeds athreshold. If this threshold is not exceeded the discharge event is notrecorded. Each partial discharge event is characterised by its peakamplitude and the point-on-wave time of occurrence with respect to the50 Hz reference. This information in digitised form is delivered to amicro-controller 36 which in turn delivers to the network node 21D. Innormal operation the micro-controller 36 is arranged to accumulate thecharacterising features of a plurality of partial discharge events fromeither a single coupler 11 or from a group of couplers 11 and to datacompress these characterising features into a single data word fordelivery to the network node 21D. By way of example, the datacompression may be by means of averaging the characterising features(amplitude and time) of a limited number of events (such as 20).

The circuit 21C also includes a self-test facility 37 which enables thecircuit to generate either directly under control of the micro-processor36 or indirectly under control of the master 25 a simulated input pulseto the peak detection and hold circuitry 31,32 to enable remoteself-checking of the functionality of the circuit.

Essentially the host 25 having established from the received data that adischarge is present in the GIS its cause requires to be identified.This is achieved by considering over a number of power frequency cyclesthe point-on-wave at which the partial discharge events occur and byconsidering the amplitude of the events. It has already been establishedthat when PD events occur around the peaks of the 50 Hz wave they arisefrom a corona source whereas if PD events occur on the rising quadrantof the positive half-cycle and the falling quadrant of the negativehalf-cycle they arise from a floating electrode and furthermore if PDevents occur essentially uniformly over the complete 50 Hz cycle theyarise from a moving particle within the pressure vessel 12. Thus thehost 25 is provided with stored data representative of these knownpre-fault conditions and with a comparator which enables a comparison ofthe pre-stored data with the communicated characterising events of anypartial discharges which have arisen. The host 25 accordingly issues awarning signal in the event of any form of identity or near identitybetween the stored known pre-fault conditions and the communicatedcharacterising events. In the absence of any identity the host outputs azero signal. Furthermore when a warning signal is issued host 25 canidentify and command the appropriate micro-controller 36 to cease datacompression and to transmit to host 25 the specific characterisingfeatures of each and every PD event either from its communicated groupof couplers 11 or from one or more selected coupler of that group.

We claim:
 1. A diagnostic measurement system for gas insulatedelectrical substations, comprising means (20) for automaticallymonitoring a plurality of UHF couplers (11) fitted to pressure vesselsin the substation, the monitoring means (20) comprising means (21B) fordetecting individual partial discharge events, means (21C) forcharacterizing detected events according to amplitude and time ofoccurrence and for representing each characterized event in a dataformat, said characterizing means (21C) being responsive to saiddetecting means (21B), means (21D) for communicating data representingcharacterized events from said characterizing means to an analyzingmeans (25), the analyzing means (25) comprising (a) pre-stored datarepresentative of known pre-fault conditions, and (b) comparative meansfor comparing the pre-stored data with the communicated data, and forissuing a warning signal in the event of identity, and means forcompressing the data representing a plurality of detected events fromeach of said plurality of UHF couplers into a single data word forcommunicating to the analyzing means (25).
 2. A system as claimed inclaim 1, wherein the analyzing means (25) comprises means for signallingthe monitoring means to communicate data representing characterizedevents from a particular coupler (11) without data compression.
 3. Asystem as claimed in claim 1, wherein the communicating means (21D)comprises an optical fiber loop (13).
 4. A system as claimed in claim 1further comprising an input protection circuit (21A) electricallyconnected between said detecting means (21B) and a respective UHFcoupler (11).
 5. A system as claimed in claim 4 wherein said inputprotection circuit (21A) comprises a co-axial stub.
 6. A system asclaimed in claim 4 wherein said input protection circuit (21A) comprisesa gas filled discharge arrester.
 7. A system as claimed in claim 4wherein said input protection circuit (21A) comprises a Schottky diode.